1. Field of the Invention
The present invention relates to a decoding circuit, and more particularly to a video signal decoding circuit, which is used in the reproducing period in a case where encoded signals are recorded in and reproduced from a magnetic recorder.
2. Description of the Prior Art
In a case where a video signal such as a television signal is recorded on a magnetic recording tape, the video signal is usually stored in the form of an analog signal. In recent years, however, owing to a rapid progress in digital circuit technology, a strong demand for a good quality of television picture and the like, digital precessing devices for a television signal have been widely employed in the field of broadcasting. In accordance therewith, there have been actively developed devices for magnetically recording a pulse-coded digital television signal in a tape recorder and for reproducing the recorded signal. The devices for magnetically recording and reproducing the digital television signal encounter a problem that the signal has to be recorded at a high density in order to store a large amount of information in a small-sized recording medium. Accordingly, the code error is inevitably increased, and the reproduced picture quality is deteriorated due to the code error. Thus, a main advantage of the digital devices for recording and reproducing a television signal is lost.
In order to eliminate such code error, coding techniques have been improved in addition to improvements in the magnetic recording medium and device. For example, when a digital television signal is recorded, there is conducted such a coding process as the addition of an error correcting (or detecting) code, the change of the sequence of codes in a code train, namely, the interlieving process, or the randomizing process for making the generation probability of a code having a level of "1" equal to that of a code having the level of "0". When a signal subjected to such a process is reproduced from a magnetic recorder, an inverse process is conducted on the recorded signal in accordance with the rule of the process in the recording period to obtain a code train corresponding to an original television signal. In many cases, the code train thus obtained is then subjected to digital-to-analog conversion to reproduce an analog television signal.
The inverse process in the reproducing period is conducted in a digital manner, and a binary code signal reproduced from the magnetic recorder is processed using a clock signal which is reproduced in the same manner as the binary code signal. Further, the above process in the reproducing period is conducted for every block formed of a code train in one horizontal scanning period, and therefore makes use of a synchronous code interposed between the blocks, as a reference.
In the above operation for magnetically recording and reproducing a digital signal, a drop-out may be generated due to dust and scratches on a magnetic recording medium. In such a case, not only the reproduced code pulses but also the reproduced clock pulses are missing from a reproduced signal. When the clock pulses are missing, the reproduction of a code signal cannot be normally conducted even at that part of the code signal where the code pulses are not missing, as will be explained later in detail. In order to prevent the absence of clock pulses even in a case where a drop-out is generated, a countermeasure has been considered in which a clock signal detected from a magnetic recording medium is applied to a resonating circuit having a high Q-value to compensate the lack of clock signal. However, since the drawing time of the resonating circuit is longer as the Q-values are higher, the resonating circuit cannot follow the jitter in the detected signal, and therefore the reproduced picture quality is deteriorated. Thus, such a countermeasure cannot bring a fundamental solution.